Boron fertilizers: use, mobility in soils and uptake by plants Fien Degryse Fertilizer Technology Research Centre Boron toxicity and deficiency in plants § Boron is an essential micronutrient required for several functions in plants, particularly for cell walls and for reproduction (flowering) § Uptake by plants is passive and unregulated, so toxicity can easily occur § B is relatively immobile in most plant species, so crops are adversely affected by even short-term deficiencies B toxicity B deficiency University of Adelaide www.acpfg.com.au www.extension.umn.edu www.agric.wa.gov.au B deficiency Boron toxicity and deficiency in plants § The window between deficiency and toxicity for B is very narrow, e.g. Gupta et al 1985, Can J Soil Sci 65: 381 § The optimal range in soil varies by crop, but is roughly 0.5-5 mg/kg hot water-extractable boron for most species University of Adelaide Boron toxicity and deficiency in plants § Species sensitive to B deficiency: legumes, Brassica, fruit trees Shorrocks 1997, Plant Soil 193: 121 § Species sensitive to B toxicity: – Several cereal species (barley, wheat) (due to low pectin content?) – Stone and pome fruits University of Adelaide Boron in soil § Total B concentrations in soil depend on parent material and the degree of weathering, with natural background concentrations usually ranging from 2 to 100 mg/kg (average ∼ 20 mg/kg; Power & Woods 1997) § Boron is never found as a single element and is usually found combined with oxygen as borates § Boron may also be tightly bound in silicate minerals to produce very insoluble minerals, e.g. clay minerals or tourmalines § Adsorption on oxides and association with organic matter occurs University of Adelaide Boron in soil § Soluble B in soil exists as a neutral species, H3BO3, in most - soils or also as B(OH)4 in high pH soils (pKa=9.2) § The adsorption of B in soils is weak, though generally higher in high pH soils (pH>8) 100 pH 9 pH 7 10 (l/kg) d K 1 0.1 1 10 100 CEC (cmolc/kg) Data from Goldberg et al 2000, SSSAJ 64: 1356 Boron in soil § The low retention makes B vulnerable to leaching ⇒ Excess rainfall can result in loss of applied B from the top layer HWB (mg/kg) 0 0.5 1 0 5 10 15 Depth (inches) 20 w 3 25 w 26 w 52 30 Data from Winsor 1951, Soil University of Adelaide Sci 71: 91 Communar & Keren 2005, SSSAJ 69: 311 Boron in soil Areas where B deficiency is most likely to occur Shorrocks 1997, Plant Soil 193: 121 University of Adelaide Boron in soil § Toxic B levels are most likely to occur in dry areas, particularly in: – soils inherently high in B – soils irrigated with B-rich irrigation water § High B is often found in association with salinity problems § The highest B concentrations are often found at depth (>20 cm), so topsoil sampling may not reveal the issue § “Detoxifying” the soil through excess irrigation is often not practical or economical, so selecting/breeding crop cultivars with tolerance is usually the only practical approach to increase or maintain yields on high-B soils University of Adelaide Boron in soil and plants - Summary § Boron is highly mobile in soil (if not occluded in minerals/ organic matter) § Boron deficiency is most commonly found in humid regions, as leaching removes plant-available B § There is a narrow range between B deficiency and toxicity, with optimal soil concentrations generally between 0.5-5 mg/ kg hot-water extractable B § The sensitivity to B deficiency or toxicity is plant species dependent University of Adelaide Boron fertilizer - Sources § Boron is primarily obtained from mined B minerals, located mostly in arid regions of Turkey and the USA, and also in Argentina, Chile, Russia, China, and Peru § Approximately 70% of the world supply comes from two corporate organizations – Eti Mine Works (Turkey) and Rio Tinto (US Borax) § It is estimated that Turkey has 72% of the world’s B reserves University of Adelaide Boron fertilizer - Sources § Soluble borates/boric acid (borax and refined borates) Compound (Common/commercial name) Formula % B Disodium tetraborate decahydrate (Borax, tincal) Na2B4O7.10H2O 11.3 Disodium tetraborate pentahydrate (Fertibor, Granubor) Na2B4O7.5H2O 15.2 Disodium octaborate tetrahydrate (Solubor, Granubor) Na2B8O13.4H2O 20.9 Anhydrous borax (Dehybor) Na2B4O7 21.5 Boric acid H3BO3 17.5 University of Adelaide Boron fertilizer - Sources § Crushed or refined ores Compound Formula % B Colemanite Ca2B6O11.5H2O 15.8 Ulexite NaCaB5O9.8H2O 13.3 Colemanite Ulexite University of Adelaide Boron fertilizer - Sources § Other sparingly soluble compounds Compound Formula % B Boron frits (boric oxide glass) 2-11 Boron phosphate BPO4 10.2 – Boron frits are produced by melting silicates with borates and have very low solubility – Boron phosphate is synthesized from boric acid and phosphoric acid and its solubility depends on the temperature of synthesis (Abat et al. 2014) University of Adelaide Boron fertilization § Recommended rates depend on B crop requirements and generally range from 0.25 to 3 kg/ha § Methods of application: – Soil application: mostly broadcast, banding not recommended because of toxicity risk – Foliar for selected crops University of Adelaide Boron fertilization - Foliar Wild Type Transgenic Brown et al 1999, Plant Phys 119:17 Brown & Hu 1998, Better Crops 82:28 § Boron is phloem-mobile in sorbitol-rich species, e.g. several fruit crops § In most species, B is not phloem-mobile and hence foliar fertilization is not effective University of Adelaide Boron fertilization § Recommended rates depend on B crop requirements and generally range from 0.25-3 kg/ha § Methods of application: – Soil application: mostly broadcast, banding not recommended because of toxicity risk – Foliar for selected crops § Types of soil-applied fertilizer: – Granular fertilizer bulk blended with granular NPK sources – Boronated NPK fertilizer University of Adelaide Boron fertilization – Bulk blend vs boronated Co-granulated product Co-granulated Bulk blend Bulk blend 62 days 7 days da Silva et al, in preparation § Bulk blending results in poorer field distribution and hence higher risk of toxicity/deficiency than co-granulated fertilizers University of Adelaide Issues with soluble B fertilizer § Soluble B may cause toxicity initially with sensitive crops § Retention of B in most soils is negligible, so B leaches easily, which may result in deficiency later in the season High concentrations Low concentrations may cause toxicity may result in deficiency Leaching loss University of Adelaide Slow release fertilizers § Slow release fertilizer reduce the risk of both leaching losses (possibly resulting in deficiency) and of seedling toxicity § The release should be slow enough to protect against leaching and harmful concentrations, but fast enough to supply the nutrients within a reasonable timeframe University of Adelaide Slow release fertilizers – Reduced leaching Borax Coarse colemanite HWB (mg/kg) HWB (mg/kg) Less leaching for coarse 0 0.5 1 0 0.5 1 0 0 colemanite 5 5 ⇒ Available B maintained at 10 10 15 15 Depth (inches) optimal levels over longer Depth (inches) 20 20 w 3 w 3 period 25 w 26 25 w 26 w 52 w 52 30 30 Winsor 1951, Soil Sci 71:99 1.2 borax ) 1 - 1 fine colemanite coarse colemanite 0.8 0.6 0.4 0.2 HWB in top 6 in (mg kg 0 0 10 20 30 40 50 60 Time (weeks) Slow release fertilizers – Reduced toxicity Seedling toxicity test (Abat et al. 2015) § Petri dish filled with soil § Fertilizer granule (MAP or MOP with 2% B) in centre § Seeds germinated under controlled conditions (23/15°C, day/ night) § Pictures taken 7 and 11 days after germination and processed using imaging analysis software (GIMP) Fertiliser granule (MAP: 30 mg, MOP: 40 mg) 10 g moistened soil and 1 g seeds 50 g moistened soil University of Adelaide Slow release fertilizers – Reduced toxicity Toxicity and HWB concentrations around MAP/MOP+2% B granules Borax = Ulex. => Colem.* > BPO4(500) > BPO4(800) Borax = Ulex. > Colem. = BPO4(500) > BPO4(800) * Colemanite more soluble due to Ca ppt University(Ca phosphates) and lower pH around MAP of Adelaide Abat et al. 2015, Plant Soil 391:153 Slow release fertilizers – More optimal nutrient supply Pot trial (Abat et al. 2015) § Sandy acid soil with low hot-water B concentration (0.2 mg/ kg) § Five B sources: borax, ulexite, colemanite, BPO4(500) and BPO4(800) co-granulated with MAP at 1% B § Canola grown for 12 weeks in 1-kg pots § After 3 months, leaching with 1400 ml of water, and second canola crop (12 weeks) grown § Yield and tissue analysis University of Adelaide Slow release fertilizers – More optimal nutrient supply First crop Second crop Borax Colem BPO Borax BPO4 4 Colem BPO4 BPO4 MAP Ulex. (500) (800) MAP Ulex. (500) (800) University of Adelaide Abat et al. 2015, SSSAJ 79: 97 Slow release fertilizers – More optimal nutrient supply § With most soluble sources, toxicity symptoms and yield reduction in first crop University of Adelaide Abat et al. 2015, SSSAJ 79: 97 Slow release fertilizers – More optimal nutrient supply § With most soluble sources, toxicity symptoms and yield reduction in first crop § With most soluble sources (borax and ulexite), deficiency symptoms in second crop University of Adelaide Abat et al. 2015, SSSAJ 79: 97 Conclusions § Boron is an uncharged ion and is not strongly retained in soils § Boron is therefore very mobile and deficiencies are most common in high rainfall environments, especially on sandy soils § The ‘window’ between deficiency and toxicity is narrow for B, so care is needed in fertilizer formulation and placement to avoid B toxicity § Only